The Emanuel African Methodist Episcopal Church in Charleston, S.C., site of one of the deadliest mass killings last year. (Brian Snyder/Reuters)

The mass killings that erupted last year seemed linked by little more than a hail of gunfire. Their locations became etched in public memory, the terror and bloodshed drawing our attention to a church in Charleston, S.C., military facilities in Chattanooga, Tenn., and most recently, a holiday gathering in San Bernardino, Calif.

But together, these violent rampages contributed to a grim statistic: At least 52 people in the United States were killed by domestic extremists in 2015, the highest number in two decades, according to a report released Tuesday by the Anti-Defamation League’s Center on Extremism.

“What a tragically noteworthy year 2015 was in terms of extremist violence,” said Mark Pitcavage, senior research fellow at the center.

More people were killed by domestic extremists last year than in the prior two years combined, and 2015 was the deadliest single year for such violence since 1995, when a federal building in Oklahoma City was bombed by men with ties to the U.S. militia movement, the report said.

The ADL linked all 52 deaths to people with ties to just four movements: White supremacists, anti‐government extremists, domestic Islamist extremists and antiabortion extremists. Pitcavage said the death toll represents the minimum possible final count for 2015, because it can take at least a year for extremist connections to emerge in some killings.

Nearly two-thirds of the 52 victims were killed in incidents directly related to these movements, where ideology played at least some role in their deaths, the report said. The rest were killed by extremists acting in ways unrelated to their beliefs. For example, the ADL counts Trevor Casper, a Wisconsin state trooper killed by a bank robber who had been involved with skinhead and neo-Nazi movements. Casper died in a shootout with the bank robber that was unrelated to ideology.

More than half the 52 deaths occurred in incidents involving multiple victims — a sharp contrast to recent years, when most killings carried out by extremists involved a single victim, according to the group’s research. And all but four of the victims died by gunfire.

“The blunt fact is that, in the past 50 years, firearms in the hands of domestic extremists have killed far more Americans than have bombs, blades, chemical or biological weapons, or any other type of weapon,” the ADL report said.

This ADL report, which The Post reviewed before its release, is the latest attempt to track and quantify the danger posed by violent extremists in the United States. New America, a Washington research center that also tallies such violence, has identified 93 deaths by “homegrown extremists” since the Sept. 11, 2001, attacks; the killings have been split about equally between right-wing attackers (48) and people inspired by jihad (45), the center says.

“There’s no question that domestic extremist violence is on the rise,” said J.M. Berger, a fellow with George Washington University’s Program on Extremism.

However, Berger said,  detailed academic research is required to determine whether these killings are “due to different kinds of extremist ideologies versus people acting out for their own reasons.”

[Terror in the American Desert]

New America’s count focuses on jihadist attacks and far right-wing attacks and does not include all of the deaths tallied by the Anti-Defamation League. Both groups included the shooting at a Planned Parenthood clinic in Colorado Springs last year that killed three people; the accused gunman later expressed antiabortion views. Neither group included the deaths of three young Muslims shot and killed near the University of North Carolina last year in their tallies. There, the accused gunman had railed against religion, sparking questions about whether the victims were targeted for their faith. But he also had a history of anger over parking issues, which investigators cited as a possible explanation for the shooting.

In recent years, law enforcement officials have grown acutely concerned about violent domestic extremism — particularly from anti-government extremists. Michael A. Clancy, deputy assistant director of the FBI’s counterterrorism division, warned in 2012 about “smaller, localized acts of violence” that could be carried out by domestic extremists, calling this threat one of the bureau’s highest priorities.

A report released last year found that law enforcement agencies said they were most concerned about the national threat posed by violent extremism. According to a 2014 survey conducted by Charles Kurzman of the University of North Carolina at Chapel Hill and David Schanzer of Duke University, 74 percent of the agencies that responded said anti-government extremism was one of the top threats in their area, nearly double the number who listed terrorist extremism.

That report was released last June and based on a survey that predated the rise of the Islamic State, a militant group also known as ISIS or ISIL. In follow-up phone calls after the group became more prominent, the officers did not change their responses, Kurzman and Schanzer wrote — though the report was released before the shootings in Chattanooga and San Bernardino ramped up concerns about terrorism on U.S. soil.

Both of those attacks were included in the ADL figures released Tuesday, and federal officials said attackers in both places appeared to be inspired by the Islamic State or other overseas terrorist groups.

“Domestic Islamic extremists are definitely one of the extremist movements causing serious problems with violence in this country right now, so we couldn’t ignore it,” Pitcavage said.

With 19 people killed in Chattanooga and San Bernardino, the death toll from attacks motivated by Islamist extremism was nearly equal to the 20 people killed by white supremacists in 2015. Each year for the past two decades, white supremacists have been responsible for more deaths than any other extremist faction, Pitcavage said. Over the past decade, killings by white supremacists have accounted for 70 percent of the nearly 300 extremist deaths tracked by the ADL.

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“For most years of the past 20 years, you’re basically looking at white supremacy and anti-government extremism,” he said. “And every now and then, there will be an act of antiabortion violence or anti-immigration violence or left-wing violence or anarchist violence or the people killed in the Boston Marathon bombings.”

Pitcavage said the death toll in these two attacks raises concerns about the role of the Islamic State going forward. The group uses social media and carefully produced videos as it “seeks to try and attract disaffected, alienated, volatile people and get them to commit violent acts in its service,” he said, adding that “2016 or 2017 may actually see similar acts.”

[Police call him an ISIS recruiter. He says he’s just an outspoken preacher.]

Pitcavage said he hopes Tuesday’s report raises a useful alarm.

“In some small way, this helps us measure the threat and understand the threat,” Pitcavage said. “If you understand something better, you can respond to it better.”

Related:

Domestic extremists have killed more Americans than jihadists since 9/11. Here’s how the government is responding.